1. Technical Field of the Invention
The present invention relates generally to the field of deterioration of products and materials as a function of time and temperature, and the determination and indication of the end of a product or material's useful life.
2. Description of Related Art
Manufacturer's stated shelf life estimates are, at best, a gross estimate of the actual state of deterioration of a product or material. The manufacturer of a product or material typically specifies the useful shelf life for predicted worst-case storage conditions. Items undergoing more benign environmental stresses may have true shelf lives up to ten times as long as those stored at worst-case conditions. As an example, paint stored at a temperature of 50.degree. C. (122.degree. F.) has a shelf life of approximately one-third that of paint stored at 30.degree. C. (86.degree. F.) based on the thermal aging constant for typical acrylic paints. The consequential damages of using products or materials such as paints and adhesives after their useful lives have expired can be enormous.
The NCEL study, conducted by Arthur D. Little in March 1993 and updated in 1996 concerning shelf life specifications ("Shelf Life Specifications for Mission Readiness; NCEL Contract Report," CR 93.001, March 1993 and CR 96.007, June 1996.) identified 55 products and materials commonly discarded prematurely because of conservative shelf life estimation by the manufacturer. Of the 55 products and materials, 16 were either plural or single component coatings or adhesives.
Product deterioration can occur as a result of a number of phenomena such as chemical aging, radioactive decay, biological aging, mechanical relaxation, and aerobic interactions which can occur during open-container periods. With the exception of radioactive decay all of these mechanisms have deterioration rates which are dependent upon time and temperature.